In the past hybrid circuits have been mainly composed of transformers connected in transformer bridge. There are bridges composed of a single transformer and bridges with two or more transformers. Most transformer hybrids employ two transformers which form two branches of the transformer bridge, the other two branches being composed of the line and a balancing network. The balancing network usually does most of the balancing because the transformers use equal windings to achieve transformer balance. The good part of transformer hybrid circuits is that the transformers provide isolation between the line and the four wire circuits, both receiving and transmitting. The line achieves a balance to ground condition by virtue of the transformer action which is otherwise hard to achieve.
In more recent years thare has been a continuous effort to achieve a simple electronic or transformerless hybrid circuit which would have all the advantages of the transformer hybrid. Most of these electronic circuits employ a balanced type of push-pull driver circuit which is balanced to ground. The first driver exhibits an impedance from each wire to ground that is balanced, but of course there is an impedance from each wire to ground which becomes a critical balance. The second amplifier is usually connected directly across the transmitting circuit, sometimes in a bridge arrangement. This compares to the transformer hybrid which is almost completely isolated from ground and which has only small interwinding capacitances to ground.
There are other new circuits, some of which employ switching devices to alternately switch back and forth between first and second branches. These switching circuits allow one branch to function while terminating the other branch. Other new circuits employ current mirrors, two transistors coupled together, to achieve some isolation and transfer. Other new circuits employ electronic switching circuits to automatically balance a bridge.
In this new invention balance to ground is achieved by complete isolation. The circuitry floats entirely above ground because one side of the line is used as the common point. The hybrid bridge is a balanced bridge with one point connected to the common line. The power for the amplifiers is derived from the line, a battery or an isolated power supply, isolating the amplifiers from earth ground because the common line wire is used as a return. Thus, it is possible to use a single ended first amplifier and a single ended second amplifier such as operational amplifiers. In this way it is possible to have a two operational amplifier hybrid circuit which is the simplest that can be obtained with amplifiers in both directions.